The files in the Moodle data folder and backup folders are referenced in the "files" table in the database. If you view the folders themselves you will find the sha1 hash code under which the file has been saved and referenced in the database. If you have any orphaned files (files no longer referenced by the database but still present on your disk) it will be very difficult to find them. This utility enables you to immediately receive a list of orphaned files together with the file type and size. You can download the file to see what it is (the name is a sha1 hash so that won't give you any clues). If you are happy to delete the file you can select it with the checkbox next to it's description and click delete. You will be asked for confirmation before the file is finally deleted.
Please note: This plugin requires the fileinfo PHP extension